Charged particle phase bunching apparatus



Dec. 19, 1967 YASUTSUGU TAKEDA ET AL 3,359,500

CHARGED PARTICLE PHASE BUNCHING APPARATUS Filed Jan. 15, 1964 'ET/ZCUUM SHIFTER I 29 40 PUMP TRANSFORMER SYSTEM II l MATCHED IMPEDANCE 22 E L3 30 LOAD J 4| 28 .VARIABLE ATTENUATOR 3s m i DIRECTIONAL MATCH ED IMPEDANCE I5 I6 MATCHED 1" COUPLER Ii. EL; \TL/MATCHED LOAD PULSE 23 8 29 PHASE SHIFTER 4i 2 MODUATOR TRANSMISSION MEANS VARIABLE ATTENUATOR I MATCHED L0A0 25 T M O D D L ATOR 34 PULSE HIGH-FREQUENCY TRIGGER SOURCE POWER MODULATOR GENERATOR INVENTORS Yamdsa u. Ta c lq,

Mfij Mukawa.

United States Patent Ofiice 3,359,5 Patented Dec. 19, 1967 3,359,500 CHARGED PARTICLE PHASE BUNCHING APPARATUS Yasutsugu Takeda, Nakano-ku, Tokyo-to, and Aki i Maekawa, Kodaira-shi, Japan, assignors to Kabushiki Kaisha Hitachi Seisakusho, Tokyo-to, Japan, a joint-stock company of Japan Filed Jan. 15, 1964, Ser. No. 337,855 Claims priority, application Japan, Jan. 18, 1963, 38/ 1,463 2 Claims. (Cl. 328233) ABSTRACT OF THE DISCLOSURE A charged particle phase-bunching device which is provided with a charged particle source including a cavity resonator, a high voltage pulse transformer to eject pulsatively charged particles from said charged particle source, an exciting high frequency wave source, and a coaxial cable which is partly wound around said pulse transformer and which is to couple said exciting high frequency wave source and said cavity resonator, thereby to supply a part of high frequency from said exciting high frequency wave source to said cavity resonator through said coaxial cable.

This invention relates to phase bunching apparatuses of the velocity modulation type used in one part of highenergy charged-particle generating means such as, for example, a linear electron accelerator. More particularly, the invention relates to a new charged particle phase bunching apparatus of miniature size having highly desirable features.

In general, the commonly known phase bunching apparatuses of velocity modulation type are those wherein velocity modulation is imparted in a concentrated manner by means of a cavity gap to a charged particle beam, which is subjected to phase bunching by means of a nonelectric-field space commonly called a drift tube, and those wherein high-frequency electromagnetic waves which impart velocity modulation in a substantially continuous manner in a certain path interval to a charged particle beam are provided, and phase bunching is simultaneously effected.

The present invention affords the same effective results when applied to either of the above cases, but the following description will be presented principally with respect to the former type of apparatus for the purposes of illustration. Furthermore, although a plurality of combinations, each comprising a cavity gap and a drift tube, can be arranged along the charged particle path, the case of two such combinations, which is considered to be readily reducible to practice and, at the same time, to have a relatively high effectiveness, is herein shown in the drawings. In addition, although it is possible to use various kinds of charged particles and various types of accelerating devices, the case of a linear electron accelerator is herein described for illustrative purpose.

It is an object of the present invention to provide a charged particle phase bunching apparatus of relatively simple construction and miniature size, which is advantageous from the viewpoint of occupied volume and economy.

It is another object to provide an apparatus as stated above which is effectively applicable to a wide range of uses.

The foregoing objects as well as other objects and advantageous features as will presently become apparent have been achieved by the present invention, which, briefly described, resides in a charged particle phase bunching apparatus of the type comprising a charged particle source,

a high-voltage pulse transformer to cause charged particles to be projected in a path from the charged particle source, and means to phase bunch, by means of high-frequency exciting waves, the charged particles so projected, which apparatus is characterized in that one portion or all of the said high-frequency driving wave energy is supplied from a high-frequency driving wave source through a coaxial cable wound around the said high-voltage pulse transformer.

The nature, principles, and details of the invention will be most clearly apparent by reference to the following description with respect to the case wherein electrons are used as the charged particles, when taken in conjunction with the accompanying drawings in which like parts are designated by like reference characters, and in which:

FIGURE 1 is a simplified diagram to be referred to in a description of the operational principle of the apparatus according to the invention; and

FIGURE 2 is a connection diagram, partly in block diagram form, showing the composition and arrangement of a preferred embodiment of the invention.

Referring first to FIGURE 1, an electron beam 1 is projected from an electron gun, the casing of which is designated by the reference character 3,, and succesively passes through a first cavity 8, a first drift tube 9, a second cavity 10, and a second drift tube 11. The various quantities associated with this apparatus will be designated by the following notations. The electrons are being injected with an injection voltage V at an injection velocity of v The first cavity 8 has an interaction gap G in which a high-frequency voltage of amplitude V is being excited, and the second cavity has an interaction gap G in which a high-frequency voltage of amplitude V is being excited. The gap G has a center point P and a gap coupling coeflicient 1 and the gap G has a center point P and a gap coupling coefiicient The phase bunching is intended to take place at a point P The points P and P are separated by a distance 1 and the points P and P are separated by a distance 1 Then, the bunching parameter X relating to the first cavity 8 and the bunching parameter X relating to the second cavity 10 are respectively given by the following equations.

where f is the frequency of the high-frequency driving wave.

For the selection of the values of X X V and V one proposal has been made (in Japanese patent applicatron No. 37/ 10,638), according to which the selection in conformance with and other conditions are close to the optimum, and it is possible to bunch at the point P approximately 90 percent of the electrons supplied from the electron gun within 10.1w radian in the vicinity of a certain phase angle.

In most linear electron accelerators, the mean electron velocity v at the accelerating tube entrance (the position P shown in FIGURE 1) is selected to be of the order of 0.5 C. for considerations of facility in design and of economy. Accordingly, in the case when the electrons are injected at a velocity of 0.5 C. from the electron gun 3, the corresponding injection voltage may be considered to be V =80 kv.

The magnitudes of the amplitudes V and V are ordinarily selected to be of such an order that the condition expressed by is satisfied in order to obtain effective velocity modulation and, moreover, to prevent the electrons from undergoing excessive velocity dispersion at the point P For example, in the case when these amplitudes are selected to be such that en V =2 kv., and en V =20 kv. (where it is assumed that e=0.8, and V =80 kv.), a design such as that indicated by the following values becomes possible through simple calculations.

=801r, l =20)t (where A is the free-space wavelength of the high-frequency wave) =41r, l

The high-frequency waves most commonly used at present are those of free-space wavelengths of the order of cm. Accordingly, the resulting distance 1 is approximately 2 meters. An arrangement wherein such a long path is placed in front of the accelerating tube of the accelerator merely lengthens the total length of the accelerator excessively and, therefore, is not desirable. This difficulty may be overcome by maintaining the mean velocity of the electrons at a low value at least until the electrons have completed their passage through the path For this purpose, when the potential of the point P is to be zero, and the potential of the cathode of the electron gun is to be -V the potential V of the first cavity 8 and the first drift tube 9 may be caused to have the relationship -V V' O.

In such an arrangement as that illustrated in FIGURE 1, the mean velocity of the electrons at least until they pass through the path I is due only to the energy component corresponding to V V'. Therefore, if the potentials V and V are so selected that, for example, V =80 kv. and V=70 kv., the electron velocity will be 0.19 C, whereby the length of the path l can be shortened to a value of the order of 1/ 2.5 of the ordinary value.

In such an arrangement, however, the excitation of the first cavity 8 at the potential V with high-frequency power becomes a difficult problem in actual practice. If, for example, the high-frequency power source is to be created in a voltage of 70 kv., the structure of the apparatus will become extremely large because of formidable problems of high potential insulation, whereby the apparatus will be disadvantageous from the viewpoint of occupied space and economy.

The specific nature of the present invention, which contemplates overcoming the above stated problems, will be apparent from a consideration of the following detailed description with respect to a preferred embodiment of the invention.

Referring to FIGURE 2, an electron beam 1 is generated in an electron gun having a casing 3, a cathode 5, a first anode 4 for drawing out the electron beam, a focusing electrode 6, and a heater 7. The casing 3 also contains a first cavity 8 and a first drift tube 9 axially disposed on the axis of the electron beam 1 and has, at its upstream end, an insulator 13 constituting the end wall and is provided with vacuum seal terminals 14.

The first anode 4 is connected to a pulse transformer 20 for the first anode, and the heater 7 and focusing electrode 6 are connected to secondary windings 15, 16, and 17 of a high-voltage pulse transformer 19, which also has a secondary winding 18 connected as shown. The said secondary windings are connected at their other ends to a heater power source 23, a bias voltage source 24 forthe focusing electrode, and a bias voltage source 25 for the first anode. Reference numerals 21 and 22 designate matched impedances connected as shown.

A second cavity 10 and a second drift tube 11 are disposed downstream from and coaxially with the aforementioned first drift tube 9. Still further downstream along the path of the electron beam 1, there is coaxially disposed an accelerator tube 2, about which focusing coils 39 are coaxially disposed. The ends of the accelerator tube 2 communicate with waveguide tubes 40, one of which on the downstream end has at its end a matched load 30 for high-frequency power. The other waveguide tube is connected at its extreme end to a high-frequency modulator 33 (such as a magnetron) and is provided with directional couplers 31 and anevacuating system 32. Lines 26 and 27 are coaxial cables, one end 'of the coaxial cable 26 entering the electron gun case 3 through a vacuumtight insulator 12. At each of the directional couplers 31, there are provided a transmission means 41 from the waveguide to the coaxial line, a variable attenuator 28, a phase shifter 29, and a matched load 30 for high-frequency power.

The high-frequency power modulator 33 is connected to its pulse source 34. The aforesaid pulse transformer 20 is connected to a pulse modulator 35 for the first anode, and the primary side of the aforesaid high-voltage pulse transformer 19 is connected to a high-potential pulse modulator 36 for the electron gun. The pulse source 34 and the pulse modulators 35 and 36 are connected to a trigger generator 37.

In the above described apparatus, a coaxial cable 26 is used to supply high-frequency waves to the first cavity 8. Moreover, this coaxial cable 26 is wound as a winding on the secondary side of the high-voltage pulse transformer 19 for the electron gun similarly as are the secondary windings 15, 16, 17, and 18, but with fewer turns than these windings. Since the center conductor and the outer conductor of this coaxial cable interlin-k With the magnetic flux established at the iron core of the pulse transformer 19, the pulsating potential assumes the desired state, and, moreover, high-frequency waves propagate in a state of coaxial mode through the interior thereof to cause excitation of the cavity 8.

That is, in the case of the above-described structure and arrangement, since the high-frequency power source is coupled through the coaxial cable to the grounded side of the secondary Winding which is the grounded side winding of the high-voltage pulse transformer, the required high potential insulation presents no problem, and the occupied space can be miniaturized, whereby economical advantage is also afforded. Furthermore, since the high-frequency power required for the excitation of the cavity 8 is, at the most, of the order of 1 kilowatt at the peak value, it is possible to take out by means of, for example, the directional coupler 31 only one portion of the power of the peak value of a few MW produced from the high-frequency power modulator 33 of the accelerator system and, after suitably adjusting this portion of the power by means of devices such as the phase shifter 29 and the variable attenuator 28, to extract this portion of the power from the transmission means 41 from the waveguide to the coaxial line.

In order to indicate the phase bunching characteristics of the apparatus shown in FIGURE 2, the following example of operation is presented. The operational conditions of potentials are selected to be as follows: the potential of the second cavity 10 and the second drift tube 11 is the ground potential; the potential of the first cavity 8 and the first drift tube 9 is pulsatingly kv.;

the potential of the cathode 5 is similarly pulsatingly 80 kv.; the effective high-frequency voltage at the point P (center of the interaction gap of the first cavity 8) is mVy=2 kv.; and the effective high-frequency voltage at the point P (center of the interaction gap of the second cavity 10) is n V =2 kv. Under these conditions, approximately 90 percent of the total quantity of the electrons are bunched within i0.11r radian about the center value of the phase at the point P (the point where phase bunching is intended to occur). Under these conditions, the required length of the first drift tube 9 is approximately cm.

Depending on the necessity, the excitation of the second cavity 10 may also be accomplished in the same manner as the excitation of the first cavity 8. Furthermore, the transformer in which the coaxial cable 26 is wound need not always be made common with the high-voltage pulse transformer for the electron gun. Although, for the wound type electron accelerator shown in FIGURE 2, an accelerator of a pulse width of approximately 5 micro-seconds and pulse recurrence frequency of approximately 300 cycles per second are commonly used, it is obvious that, in the practice of this invention, an accelerator operated by pulsating high voltage selected from a wide range of general types and specifications can be used.

In order to indicate still more fully the nature and utility of the present invention, the following summary of its effectivenesses and unique features is presented.

In general, in the phase bunching of charged particles by means of an apparatus operating intermittently according to the invention, the potentials of a space which imparts velocity modulation to the charged particles and of a drift space in which the velocity modulated particles are changed into density modulated particles are suitably adjusted, and the mean direct-current velocity of the particles traveling through these spaces is lowered. By this operation, it is possible:

(1) To shorten the path length of the drift space and thereby to reduce the size and weight of the apparatus;

(2) To reduce the consumption of high-frequency power for accomplishing velocity modulation and thereby to contribute to the economy of the operation;

(3) To reduce excessive deviations of kinetic energy of the phase bunched particles.

Accordingly, the present invention is applicable not only to charged particle accelerating devices as mentioned above but also to high-frequency electron tubes such as, for example, multi-cavity klystrons and resnatrons, as well as other devices. That is, for example, the essential constructional elements of a multi-cavity klystron are, at least, an input cavity for velocity modulation of electrons, a drift space for phase bunching the velocity modulated electron beam, and an output cavity for extracting, as a high-frequency output, the kinetic energy of the phase-bunched beam. The present invention is applied to a klystron by causing the potential of the input cavity and, further, of the drift space to be negative relative to the output cavity potential at the time of operation, causing phase bunching of the electrons therein while attaining the advantageous effects 1, 2, and 3 set forth above, imparting kinetic energy to the electron beam through utilization of the potential difference established between the output cavity and the drift space, and causing the beam to pass through the output cavity. In the case of a klystron, the aforestated effect 2 is valuable for increasing the gain and raising the efiiciency.

In the case of a resnatron, the part between the cathode and the control grid is the part wherein the electrons are phase bunched and must have a potential differing essentially from that of the output gap as mentioned in Chapter 6 of Theory and Application of Microwaves by A. B. Bronwell and R. E. Beam. In this case, a difficulty common to the aforementioned klystron and the resnatron is the problem of how to supply high-frequency Waves to the phase bunching section, that is, to the input cavity section in the case of a klystron and to the region between cathode and the control grid in the case of a resnatron. Accordingly, the present invention is admirably suitable for such applications to afford the same effective results as in the aforedescribed case of supplying high-frequency waves in a charged particle accelerator.

As is apparent from the foregoing disclosure, the present invention affords, through a relatively simple construction and arrangement, miniaturization of charged particle phase bunching apparatuses which are highly advantageous from the point of view of occupied space and economy.

It should be understood, of course, that the foregoing disclosure relates principally to only a preferred embodiment of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. In a charged particle phase bunching apparatus provided with a charged particle source including a cavity resonator, a pulse transformer to cause charged particles to be projected from the said charged particle source, and means to cause the charged particles so projected from said source to be phase bunched by exciting highfrequency electromagnetic waves, the combination of a coaxial cable having one part thereof wound about the core of the pulse transformer, an exciting high-frequency wave source, and means to supply at least one portion of the power of the said exciting high-frequency electromagnetic waves from the said exciting high-frequency wave source through the said coaxial cable to said cavity resonator.

2. A charged particle acceleration device comprising: a charged particle source including a cavity resonator; a high voltage pulse transformer to pulsatively project charged particles from said charged particle source; an exciting high frequency wave source to generate high frequency waves for accelerating said charged particles; means to accelerate said charged particles by the action of said high frequency waves; a coaxial cable, a part of which is magnetically coupled with said high voltage pulse transformer; and means to supply a part of said exciting high frequency wave output to said cavity resonator through said coaxial cable.

References Cited UNITED STATES PATENTS 2,543,082 2/1951 Webster 315-5.42 2,888,596 5/1959 Rudenberg 3155.41 2,920,228 1/1960 Ginzton 31363 2,993,141 7/1961 Post 3l5-5.42

HERMAN KARL SAALBACH, Primary Examiner. S. CHATMON, JR., Assistant Examiner. 

1. IN A CHARGED PARTICLE PHASE BUNCHING APPARATUS PROVIDED WITH A CHARGED PARTICLE SOURCE INCLUDING A CAVITY RESONATOR, A PULSE TRANSFORMER TO CAUSE CHARGED PARTICLES TO BE PROJECTED FROM THE SAID CHARGED PARTICLE SOURCE, AND MEANS TO CAUSE THE CHARGED PARTICLES SO PROJECTED FROM SAID SOURCE TO BE PHASE BUNCHED BY EXCITING HIGHFREQUENCY ELECTROMAGNETIC WAVES, THE COMBINATION OF A COAXIAL CABLE HAVING ONE PART THEREOF WOUND ABOUT THE CORE OF THE PULSE TRANSFORMER, AN EXCITING HIGH-FREQUENCY WAVE SOURCE, AND MEANS TO SUPPLY AT LEAST ONE PORTION OF THE POWER OF THE SAID EXCITING HIGH-FREQUENCY ELECTROMAGNETIC WAVES FROM THE SAID EXCITING HIGH-FREQUENCY WAVE SOURCE THROUGH SAID COAXIAL CABLE TO SAID CAVITY RESONATOR. 